Acetaminophen d4

Legally exempt, certified reference standards (fentanyl, carfentanil oxalate, acrylfentanyl, 4‐fluoro‐isobutyryl fentanyl (FIBF), furanylfentanyl, heroin, acetaminophen, chlorpheniramine maleate, and pseudoephedrine) and isotopically labeled internal standards (fentanyl‐d5, carfentanil‐d5 oxalate, acetaminophen‐d4, chlorpheniramine‐d6 maleate, and pseudoephedrine‐d3 HCl) were purchased from Cerilliant Corporation (Round Rock, TX, USA). Caffeine and Caffeine‐d3 were purchased from Sigma‐Aldrich (St. Louis, MO, USA). The structures of all drugs used in this study are given in Figure S1. HPLC grade methanol was obtained from VWR International (Mississauga, ON, Canada). ACS grade formic acid was acquired from GFS Chemicals (Columbus, OH, USA). Deionized (DI) water was prepared using a water purification system (18.4 MΩ·cm Facility Scale Reverse Osmosis/Ion Exchange Water Purification System; Applied Membranes Inc, Vista, CA, USA). All standards were prepared gravimetrically, with w/w concentrations represented as ng of analyte per g of total mass.